Regulatory mutants which affect attenuation of the threonine (thr) operon will be isolated and studied. Mutants will be isolated by two separate approaches. Firstly, using classical lactose (lac) operon genetic techniques, mutants will be selected in strains which carry genetic fusions of thr operon enzymes and the lacZ gene (beta-galactosidase). By employing various selection techniques originally developed for the lac operon, thr regulatory mutants, which overproduce or under-produce beta-galactosidase activity, will be isolated. Secondly, procedures using in vitro mutagenesis techniques will be employed to direct mutations into specific sites in the thr regulatory region. Mutants will be characterized both by their regulatory effects in vivo and by DNA sequencing. It is anticipated that many mutations will lie in regions of rotational symmetry which code for RNA secondary structure. The positions of the mutations and their effects on regulation will supply information on the relationships between secondary RNA structure and transcription termination.